The present invention relates to a tracking-servo device for use in a recorded information readout device, and, particularly, to a tracking-servo device for effectively controlling the track jumping operation of a information pick-up detecting point during the so-called random access period for the recorded information search on the recording medium.
In a reading device for reproducing information recorded on a recording disc such as video disc or digital audio disc, the so-called tracking-servo system is utilized since the information pick-up detecting point must always trace recording tracks even if the recording tracks are eccentric. Further it is usual that, in reproducing such discs, there is provided a random access function for an information search, and thus the information detecting point is driven substantially orthogonally to the tracks to realize track jumping control.
A schematic block diagram of such a device is shown in FIG. 1 as applied to an optical information reading device. In the same figure, a reading light spot 2 which defines the information detecting point is projected onto a recording track 1 of a recording medium, and reproduction of the information is performed using the reflection of the spot 2. In addition to the spot 2, a pair of light spots 3 and 4 are projected onto the medium such that the light spots 3 and 4 are positioned on opposite side edges of the track 1, respectively, when the spot 2 lies on a center line of the track. Reflections of the light spots 3 and 4 are detected by light receiving elements 5 and 6, respectively, and the difference between the output of the elements is obtained by a differential amplifier 7. The output difference (C) represents the orthogonal deviation of the information reading light spot 2 from the recording track 1. That is, the so-called tracking error signal is obtained from the output (C) of the differential amplifier 7.
The error signal (C) is supplied through an equalizer 8 and a loop switch 9 to a drive amplifier 10. An output of the latter amplifier is used to drive a tracking actuator for shifting the information detecting point 2 orthogonally to the track. The actuator, for example, is composed of a coil 11 and a tracking mirror 12 rotated by the coil in operation. Current flowing through the coil 11 is detected by a resistor 13 and an output from the resistor is fed back to the amplifier 10 to stabilize the system.
In addition to the tracking-servo system described above, a jumping operation control system is provided to make a random access possible. After any noise component of the tracking error signal (C) is removed by a low pass filter (LPF) 14, it is supplied to a comparator 15. The comparator 15 is a so-called zero level comparator, and an output (D) of the comparator 15 is supplied to respective inputs of switch controllers 16 and 17. An output (E) of the controller 16 is used to control the loop switch 9 in an on-off manner. On the other hand, the switch controller 16 is controlled by an output of the controller 17, which has a jump instruction signal (A) as one of its inputs. A jump-drive circuit 18 is actuated by the other output of the controller 17 to produce a jump-drive and jump damping waveform (F), which is supplied to the input of the amplifier 10.
FIG. 2 shows the operational waveforms in the device in FIG. 1, in which FIGS. 2A to 2F show the waveforms of the respective signals (A)-(F) indicated in FIG. 1. When the jump instruction signal (A) is supplied to the controller 17, a signal output (B) is provided at the output thereof. In response to the timing of the output (B), the output (E) of the switch controller 16 is changed from a high level to a low level as shown in FIG. 2E, causing the loop switch 9 to turn off. Therefore, the servo loop is opened when the level of the control output (E) is low. The jump-drive circuit 18 is actuated by the instruction from the controller 17 to produce a jump-drive waveform (F) shown in FIG. 2F, which is supplied through the amplifier 10 to the drive coil 11 forming a part of the actuator.
Accordingly, the tracking mirror 12 is rotated to thereby move the spot 2 orthogonally to the track, to thereby produce an error signal (C) (FIG. 2C) from the amplifier 7. The error signal amplitude initially increases up to a peak valve and then decreases, and reaches zero when the spot 2 arrives around the center of the recording track. At the moment when the error amplitude becomes 0, the output of the comparator 15 changes its state as shown in FIG. 2D. In response to this change of the output (D), the controller 17 is actuated to cause the jump-drive circuit 18 to produce a jump damping waveform F to restrict movement of the actuator. At the same time, the output (E) of the switch controller 16 is changed from a low level to a high level to turn the loop switch 9 on to thereby complete the servo loop.
In this manner, the information reading spot 2 is moved to an adjacent track to complete the jump operation. The waveform (B) in FIG. 2B is used to make the switch controller 16 inoperative so that it does not detect a zero crossing point of the negative going portion of the waveform (D) until the waveform (D), which is the output of the comparator 15, becomes settled.
In this case, the tracking actuator is supported by a resilent member or a viscous-resilent member so that a reaction or restoration force proportional to the amount of deviation of the actuator from a mechanical neutral position thereof is exerted thereon. When reproduction of a recording disc having a large amount of eccentricity is desired, the deviation of the actuator from the mechanical neutral position thereof becomes corresponding large. Where a jump operation should be performed under such a condition, the drive signal (F) from the jump drive circuit 18 flows through the coil 11 when the servo loop switch 9 of the device in FIG. 1 is turned off, and, simultaneously, a large restoration force may act on the largely deviated actuator just before the turn-off of the loop switch. Since the direction and amount of the restoration force are determined by the deviation angle and direction of the mirror 12 which has traced a desired recording track immediately before the commencement of jump, it is hard to determine the values thereof. Therefore, due to the restoration force having uncertain factors, the acceleration of spot movement when the servo-loop is opened may not be constant and it may be difficult to obtain a stable jump operation. This is particularly true in the reproduction of information from a disc having large eccentricity.